It is known in the art, as described e.g. in U.S. Pat. No. 5,424,548, that an e-beam system must be corrected for systematic or constant errors that arise because of various non-linearities in the system. For example, FIG. 1 shows in highly simplified form, an e-beam system in which a wafer 30 has a pattern being written on a field (or small portion) 31. FIG. 2 shows the wafer in more detail as being divided up into a set of fields 31. A field represents the deflection range of the beam. Mechanical motion of the stage is used to move from one field to another. Those skilled in the art will be aware that distortions will increase non-linearly as the distance from the system axis increases and fields 31 at the edge of the wafer will have more distortion than those close in. Within a field, distortions also increase as the beam deflection increases.
It is standard practice to make a calibration wafer by writing a stock pattern, such as that shown in FIG. 3, measuring the wafer after transferring the pattern to the wafer, and generating an algorithm to convert the nominal data (if the fields in the system were perfect) to corrected data. Such corrected data will be referred to as global correction data and will depend on system distortions, not on the density or other features of the pattern being written. Conventional measuring systems, such as the LMS 2000 by the Leica company are commercially available to perform the measurement. Those skilled in the art will be aware that there is another class of errors that vary from one mask to another and that depend on at least the density of lines within a field 31.
The cited patent discloses an approach to correcting such errors by adding metrology features or marks, such as those shown in FIG. 3, in vacant portions of the particular mask in question. Thus, the distortions in the metrology marks located within the nth field (31n) will depend on the mask pattern in that field 31n. A sacrificial calibration wafer is made up containing the metrology marks and is discarded after the measurements for correction have been made.
As the art has progressed, the requirements for distortion correction have become more stringent and the density of features has increased, so that the previous method is no longer sufficient.
The previous method, moreover, not only made a sacrificial mask that was discarded, it also required two computer runs to generate the e-beam control data for the two masks. The computer resources used in the "post-processing" programs used to generate the e-beam data are non trivial. For example post processing for a complex state of the art microprocessor can take tens of hours on a powerful machine such as an IBM R/S 6000 mod 595. Since mask shops make masks in only small quantities, this expense cannot be spread over a lengthy production run and has a significant effect on the shop's costs.
Thus, the field of mask making has sought an economical method for obtaining pattern-dependent corrections. The field has also sought a method of reducing turn-around time in mask fabrication by reducing post-processing time.